Photocathode



PHOTOCATHODEI E. E. SHELDON Original Filed Nov. 5, 1948 March 16, 1954Reissued Mar. 16, 1954 assez rno'rocsrnons Edward Emanuel Sheldon. NewYork, N.Y. origins: No. 2,603,751, dates .my 15, 1952. serial November5, 1948. Application for reissue July 13, 1953, Serial .Ultima (Cl.313-101) Matter enclosed In heavy brackets I: j appears in the mantenuti:umanamente reissue specication; matter printed in italics indicates theadditions made by reissue.

This invention relates to novel type of photocathodes for eiiicientpicking up oi signals or complete images produced by dinerent types ofinvisible radiations and to novel type of electrodes for intensincationoi' said signals or images produced by invisible radiations.

The purpose o! my invention is to provide photocathodes which willemciently respond to invisible iight radiation. as well as to atomicparticles radiation.

Another purpose of this invention is to provide photocathode's i'orconversion oi signals or images oi one type oi radiation into anothertype oi' radiation.

Another 'purpose of this invention is tc preserve ndelity ,oireconverted signals or images in relation to the original signals orimages.

Another purpose oi this invention is to provide a universal electrodefor intensication oi' signals or images. which ls responsive to alltypes or radiation.

The present photocathodes are made oi photoemissive, photo-conductive orof photo-voltaic layer on a suitable supporting base. Thesephotocathodes responded well to visible light. They are. however. veryinsensitive to intra-red radiation oi' wave-length longer than onemicron andon the other end of the spectrum to rays oi' wavelengthshorter than 2000 A. These photocathodes are also completely insensitiveto electron or other atomic particles radiation. Limitations oi thepresent photocathodes were eliminatedin my invention by the use oicomposite photocathodes comprising in combination light reiiecting layertransparent to exciting radiation, fluorescent layer. light transparentlayer and photo-sensitive layer disposed in a suitable vacuum tube. Thisnovel photocathode may be used in every Bienal or image reproducingSystem. as we ll as in every signal or image transmitting system suchas. phototubes, electronmultiplier tubes. in imageA converters, in imageampliilers. and in television pick-up tubes. By using this novelphotocathode the invisible exciting radiation is converted in theiiuorescent layer oi' said composite photocathode into iluorescent lightof wavelength to which the photosensitive layer of said photocathode ismost sensitive. The fluorescent light is exciting said photo-sensitivelayer directly and by reiiection from said reflecting layer, whereby anyloss of fluorescent light is prevented increasing markedly theefiiciency o! Ythis novel photocathode. In this way radiation whichwould be too weak to excite the conventional photocathode may nowproduce photo-electric eilect. The sensitivity ot the novel photocathodeis further increased by elimination oi' the optical system forprjectionoi' the signals or images on the photocathode. In the conventionalphotocathode the signals or images after their conversion into type oiradiation to which said photocathode is sensitive. have to be projectedthereon by means ot optical system. The use oi' the optical systemcauses loss of of radiation because of absorption. Elimination of theoptical system would obviously result in complete deterioration oi' thesharpness oi' projected image on the conventional photocathode. oiillumination is removed without impairing sharpness oi the image,because of close apposition ot iiuorescent and photo-sensitive layers.This is equivalent to 20-30 fold gain in sensitivity oi' thephotocathode.

Another important feature oi' the novel photocathode is the presence oitransparent .layer be tween the iiuorescent and photosemitive layers.This separating layer being extremely thin. oi' a few microns only, doesnot cause any deterioration of sharpness and at the same time preventschemical interaction ot uorescent and photosensitive layers which isfthe cause of serious complications such as spurious signals. hl. etc.

The photo-electric signals or images produced by the novelphotocathodecan be further intensitled by the use of similar compositeelectrode comprising light reflecting layer, electron iluorescent layer.light transparent separating layer and photo-electric layer, disposed ina single' or plural successive stages in the vacuum tube in cooperativerelationship with said photocathode..

The invention will be better understood when takeninconnection with thedrawings.

In the drawings:

Fig. 1` represents composite photocathode responsive to electronradiation.

Fig. 2 represents composte photocathcde sensitive infra-red radiation.

Fig. 3 represents composite photocathode re sponslve to ultra-violetradiation.

Fig. 4 represents composite photocathode responsive to X-rays and atomicparticles radiation 1 Fig. 5 represents composite photocathode sensitiveto X-ray and gamma radiation.

Fig. 6 represents an alternative form ot photocathode sensitiveinfra-red radiation Fig. '7 represents an alternative form oi com- In myinvention. this cause o! the lo` 3 posite photocathode sensitive toX-ray and atomic particles radiation Fig. 8 represents compositephotocathode responsive to infra-red having photo-sensitive layer ofphoto-voltaic type.

Fig. `9 represents novel composite electrode.

Fig. 10 represents composite electrodes arranged in successive stagesfor image amplification.

Fig. il represents cascade i'orm o! composite electrode.

Fig. l represents composite photocathode l hav-` ing light reectinglayer 2, fluorescent layer l, extremely thin light transparentseparating layer I. and photo-sensitive layer e. g. ot photo-emissivetype 5. This photocathode is the moet suitable for atomic particlesradiation such as electrons 6 and for invisible electrode-magneticradiation o! wave-length shorter than 2000 A. The electron radiationpasses through light reflecting layer 2 ot aluminum. is .striking Vthefluorescent layer I ofZnSAg or i' B'aBOi and is converted therein intofluorescent light which is exciting directly and by reflection from thereilecting layer 2 the photo-emissive layer 6 oi caesium. lithium orpotassium on antimony or bismuth. The transparent separating layer l maybe ofmica, silicon.

or of a suitable plastic. The fluorescent materialx auch as 'ZnSAg orBeso; have the property of emitting also ultra-violet luminescencebesides the visible luminescence. ZnFlz is transparent to ultra-violetradiation. Silicon and ZnFla crematerials which can be evaporated. Thiscomposite photocathode is characterized by marked sensitivity. It is 200times more sensitive than the conventional photocathode to irradiationby atomic particles or light of wave length shorter than 2000 A. At thesame time because of close proximity of fluorescent and photo-emissivelayers, the conversion o! signals and images is possible withpreservation of the sharpness, in spite oi the elimination oi opticalsystem for pro- Jection o! said signals or images on said photocathode.

Fig. 2 represents composite photocathode i0 suitable for infra-redradiation. The iluorescent layer 'I may be oi' alkaline earth sulphidesor selenides activated by cerium, samarium or by CuPb. This nuotescentlayer converts the infrared signals I0a into iiuorescence of 60G-,1000miliimicrons wave length. which is exciting the photoemissive layer l ofCsOAg or of CsO and is producing pbotoeleotron emission. The transparentseparating layer I may be o! mica, silicon. or oi a suitable plastic.This composite photocathode is responsive to intra-red signals or imageswhich would not be able to activate any known at present photocathode.

Fig. a illustrates composite photocathcde I I sensitive to ultra-violetradiation Ila. The iluorescent layer l! is of calcium phosphate withactivators. or or calcium silicate or barium silicate with activators.The light transparent separating layer Il is of mica. silicon or asuitable plastic. The photoelectric layer il is of caesium or potassiumon antimony. The iluorescent layer I2 when excited with short U.-V.radiation Ila converts it into ultra-violet fluorescence ot'3,0003,800A. wave-length, which is able to excite the photoelectric layer il. Theadvantage or this novel photocathode is, besides its sensitivity, thetact that it is responsive to very short ultra-violet radiation. towhich none of the present photocathodeslssensitive.

4 is sensitive to X-rays and atomic particles radiation. photocathodecomprises light reflecting layer Il such as, of aluminum. iiuorescentlayer i1 of ZnSCdSAg, BaBOg. or o! tungstate. The separating layer Il islight transparent and may be oi mica. silicon, Znllz or of suitableplastic. 'Ihe photo-electric layer il is of photo-conductive type andmay be of selenium. ZnB. CuS. Phs or of thallium sulphide. The signalplate 2011s ot conductive metal. The invisible X-ray radiation Il isconverted into uorescent light in the layer II. The iluorescent iightirradiating the photo-conductive layer I9 changes its resistanceproducing thereby electrical signals modulated by the pattern oi' saidinvisible X-ray image. The electrical signals now through the signalplate 2l to appropriate receivers.

Fig. 5 illustrates an alternative form ot the photocathode shown in theFigure 4. In this embodiment of the invention the photocathode lia.comprises liglt r'eecting layer I6, iiuorescent layer I'I. lighttransparent layer It. photo-conductive layer Il, another iluorescentlayer Ila and signal plate 2l. This type o! photocathode is verysensitive to X-rays and gamma rays. The photo-conductive layer l! is inthis form o! invention irradiated by the uorescent light trom bothsides, from layer II and Ila, producing double photo-conductive eil'ect.

Ilig.` 6 illustrates variety of photo-conductive photocathode 2lsuitable for infra-red radiation 28. The iluorescent layer Il is ofalkaline earth sulphides or selenides activated by' cerium, samarium oreuropium. The separating light transparent layer It is ot mica. siliconor ot suitable plastic. The photo-conductive layer 2li is of selenium.Z118, CuB. PbB or oi thallium sulphide. The signal plate of conductingmetal 21 serves to transmit electrical signals modulated by intra-redimage to appropriate receivers.

The photo-conductive composite photocethode may be also made sensitiveto ultra-violet radiation by making the fluorescent layer 24 ot saidphotocathode 2l of ZnBAg. of BaSOi or of calcium or barium silicateswith proper activators.

Fig. illustrates photo-voltaic type of composite photocathode IB havinglight reilective layer 29, for example. o! aluminum, iluorescent layerof ZnB, CdSAg, germanates or of ZnSCu I0, light transparent layer ofmica, silicon or of suitable plastic Il. photo-voltaic layer ot CuzO onCu 82 and signal plate I! ci conductive metal. The in- `visiblegX-reyradiation M is converted in the fluorescent layer Il in the nuorescentiight which is exciting the photo-voltaic layer Il directly and byreflection from the reiiecting layer 2l causing diilerences in potentialover the surface of said photo-voltaic layer. dlfierenceg in pgtentialhave the pattern of the original invisible X-ray image. They areconducted in the form of electrical signals by the signal plate u toappropriete receivers.

Fig. 8 shows an alternative form o! photovoltaic photocathode which issensitive to radiation o! long wavelength such as infra-red Ila. Thiscomposite photocathode Il consists ot iluorescent layer 3i of alkalineearth sulphides or selenides activated by cerium. samarium or europium,of iight transparent layer 36 ci mica., silicon or oi' suitable plastic;of photo-voltaic layer 3l of CuaO on Cu and of metallic signal plate3.8. The intra-red radiation is converted by the nuorescent layer ofsaid photocathode into fluorescent light which exciting said photo- Fig.4 shows composite photooatlwd l! which 75 voltaic layer causesdinerences in potential over iis surface. These dillel'ne in potentialhave the pattern or original infra-red image. They are converted intoelectrical signals and are conducted by the signal plate to receivers.

Fig. 9 illustrates composite electrode lil in vacnum tulle l forlntenslncatlon of signals or images. This novel electrode can be usedfor signais or images oi' all types oi' radiation. It has lightreflecting layer 4l, which is ltransparent to electrons, layeriluorescent under electron irradiation lig-light transparent layer 3 andphotoemissivel'layer 44. In particular the light reflecting layer Il maybe of aluminum, the electron uoresceht layer l2 of ZnSAg or of BaSG4,the light transparent separating layer 43 which may be of materialsdescribed above for the light #emparent separating llayer 4, thephotoemiaeioe layer 44 may be of caesium, potassium or lithium onantimony or bismuth. As images ot all types of invisible radiation maybe converted by a suitable compos-ite photo-emissive phoioeathode, asdescribed above. into photoelectron image. the novel electrode which iselectron-sensitive may serve for intensiilcation o! signals and imageswhether they are produced by U.V.. I-R, gamma rays or by atomicparticlesa.,

In the Flglire 9, we see intensication oi' electroiig image by saidcomposite electrode I5. The elec- `tron beam Il carrying the invisibleelectron image, such as e. g. radar image is focused on the compositeelectrode 45.' The electron image passesthrough the `light reilectinglayer 4|. is converted by the ilucrescent layer l2 into nuorescent imageand said fluorescent image is exciting photo-emisslve layer M, wherebyintensifledphoto-electron image navi-ng the pattern of the originalradar image is obtained and may be now reconverted into .visible image.if so desired This process of intensincation may be repeated a few timesusing said composite electrodes in a few stages. whereby additionalintensincation is obtained. In ease el radar images the fluorescentlayer in the composite electrode 46 should be in some instances oi' along persistence. in. order to avoid `the flicker. Such persistentfluorescent phosphor may be oislngle layer type suchv as e. g.Zn(Mg)Fa:Mn or other iluorides. ZnBCu(Agl; CsaPeOrxDy or Z'nBCdBmmCu.Sometimes it is more advantageous to use cascade type or fluorescentscreen consisting of two or more layers, as illustrated in theFigm-e 1l.In this .form of invention the composite electrode "a has thefluorescent layer a comprising two layers 12b and llc such .as e. s.ZnBlAg) on ZnSiCdkCu or A120; on ZnSzfAg). Obviously there are manycombinations of cascade long persistent fluorescent screens which can beused in this invention. The remaining parts of the composite electrode asuch as light reflecting layer Ila. light transparent separating layer aand photo-emlssive layer a are as described above.

The application of the composite electrode 4l for visible light imageamplication is shown in the Figure 10. The visible light image Il pro-Jected by the optical system Il causes photoemissive etl'ect in thephotocathode 4l disposed in the vacuum tube Il. The photo-electronsreleased irom the photocathode 48 and having pattern of said light imagestrike the composite electrode 45, and produce intcnsiiied iluorescentimage in the iluorescent layer I2 oi said composite electrode. Thelntenslned fluorescent image directly and by reilection from the lightrelayer 4I, producing intensined secondary photoelectron image. Thisintensified secondary photoeleetron image having the pattern of theVoriginal light image may be projected on the `nest composite electrodeil having llsht reilecting layer il. electron-uorescent layer u. lighttransparent separating layer ll and photo-emissive layer 5G wherebyadditional intensiilcation is achieved. This process may be repeated 'ina -ftew stages resulting in marked intensification of the original lightimage. lbefore reconverting said intensined secondary photo-electronimage-into ilnal visible image for inspection or recording. This syste-mot light image amplification was never accomplished successfully beforeas in previous systems the absence of the lightreilccting layer 4| inthe composite electrode caused back-scattering o! the fluorescent lightAfrom the fluorescent layer 42 to the phctofc'athode 4I, destroyingthereby completely the image.

It is obvious that thenovel composite photocathode and compositeelectrode may be' Aused in every signal or imago reproducing ortransmitting system. InV particular they willbeused in photo-tubes.electron multipliers. image oonverters, image empl-mer tubes as well astelevision pick-up tubes. whenever the presently known photo-cathodesare not responsive or not sensitive enough to the depicting radiation.`A

Although the preferred embodiments of theinvention have been describedit will laborious to those skilled in the art that various changes andmodiilcations may be made without departing from the true` spirit andscope ol this invento Vprotect by Letters Patent of the United States: Yl. In a vacuum tube. a composite screen com-.- prlsing a luminescentlayer deposited on a wall of said tube. a light transparent separatinglayer in contact with said luminescent layer, said light transparentlayer having a thickness oi' the order ot microns, and a photosensitlvelayer in contact with said light transparent layer.

2. In a vacuum tube.- as delinea in claim` 1 wherein said photosensitlvelayer is photoemissive.

claim l.

-' tion.

What I claim and Want 3. In a vacuum tube. as dened in wherein saidluminescent layer comprises a plurality or layers of differentphosphor-s.

4. In a vacuum tube. a composite screen comprising a luminescent layerdeposited on a wall of said tube, va light transparent separating layerof the order of microns ln contact with said luminescent layer, andvaPhotoconductive layer in contact with said separating layer. 4

5. In a tube, a composite screen comprising a luminescent layer, alight`transparent separatino layer in contact with said luminescent layer,said light transparent separating layer having a thickness of the orderof microns, and a photo.- electrie layer in contact with said lighttransparent layer.

6. In a tube as defined in claim 5, wherein said Photoelectric layer isphotoemtssive.

7. In a tube as dened in claim 5, wherein said luminescent layercomprises a plurality of layers of allier-ent phosphore.

8. In a tube, a composite screen comprising a luminescent layer. a lighttransparent separating layer of a thickness of the order of microns andin contact with said luminescent laver, and a photoconductive layer incontact with said separating layer.

9. In a tube, a composite screen comprising ilecting layer Il isexciting the photo-emissive luminescent means, light transparentseparating -ent means and the sapos means in contact with saidluminescent means and also ,transparent to ultra-violet radiation`emitted bg said luminescent means, said lights transparent means havinga thickness of that` order of microns, and a. photoelectric lager incontact with said transparent separating means 10. In a vacuum tube, acomposite screen coms prising in the following order: a luminescentlager receiving an image for-ming radiation, cg light transparentseparating lager also transpar entf'to ultra-violet radiation emitted bgsaid? luminescent layer, in contact with said luminesq cent layer and ofa thickness of the order of microns, and a photoemissioe Alager incontact with said light transparent lager.

11. In a tube, a composite screen comprising in the following order:luminescent means re-'s ceiving an image forming radiation, lighttrans-` parent separating means of evaporabie materia 1 and in contactwith said luminescent means, sai light transparent means having athickness of the order of microns. and photoelectric means` receivinglight from said luminescent mean through said light transparentseparating mea and in contact with said light transparent pieanss` 2:,

12. In a vacuum tube, a composite screen com prising in the followingorder: a lumines lager receiving an image forming radiation, lighttransparent separating layer of euaporable material in contact with saidluminescent layer and of a thickness of the order of microns, 'and aphotoemissive lager in contact with said light transparent lager.

13. In a tube, a composite screen comprising luminescent means, lighttransparent separating means in contact with said luminescent means,`said light transparent means having a thickness of the order of microns,and a photoelectric layer having one surface in contact with saidlight-1 transparent means and the other surface ex, posed,

14.V In a tube, a composite screen comprising luminescent means, lighttransparent separating means in contact with said luminescent means andof eoaporable material, said light transpar-f ent means having athickness not exceeding a few microns. and a photoelectric lager having,one surface in contact with said light transpar-l other surface exposed.

15. In a tube. a composite screen comprising in the following order:luminescent means re-v` ceiving image forming radiation, light transparent separating means of eoaporable material ir contact with saidluminescent means and of a thickness of the order of microns, andphoto-fw 5- electric moans m contact with said nani trans-" parent meansand comprising an alkali metal combined with an element of the groupincluding@J antimong or bismuth. ,f

16. In a tube, a composite screen comprising 60 X -raysi luminescent,`:means supporting means transparent to Xirags, lumi. nascent means saidX-rags, light transparent separatinqemeans in contact with saidluminescent means and of a thickness of the order of mignons, andphotoelectric means hauing one in contact with said transparentseparating .ns and the other surface exposed.

17. In a vacuum tube; a composite screen comprising in the followingorder: supporting means transparentV to Xssaps, luminescent meansreceioing said X-razls, iighttransparent separating means of eoaporaoiematerial in contact with said luminescent means and o! a thickness ofthe order of miercns, and a photoemissive lager in contact withsaid'tsamarent separating means.

18. In a tube, a composite screen comprising in the following onder.lsupporting means transparent to X-rapphlminescent means receiving saidX-rags, light transparent separating means of eoaporable material incontact with said luminescent means and "of a thickness of the order ofmicrons, anhaphotoaonductive lager in contact with said tmsparent means.

19. A tube comprising a composite screen having in the ollowingsfsorder:supporting means transparent to X-rags, luminescent means receioing saidX-rags, isg'lt transparent separating means of eoaporahle aterial incontact with said luminescent means and of a thickness of the order ofmicrons. and a pIwtoGlectric layer having one surface in contact withsaid light transparent means and the other surface exposed.

20. In a tube, a composite screen comprising in combination supporingmeans transparent to receiving said X- rags, light tran `t separatingmeans also transparent to altrasiiolet radiation emitted bg saidluminescent in contact with said luminescent means-fan of the thicknessof the order of microns, and photoelectric lager having one surface ,incontact with said transparent separating mms and the other surfaceexposed.

Ewsnp :MANUEL sHEmoN.

References qitedin the 111e of this patent ortls 9171853191 paientUNITED STATES PATENTS Nambe:79 Nxlxlx Date 2,198. 4 Apr. 23, 19402,200,853 Il-Jggsexxg'et al. May 14, 1940 2.233.296 Law .i Mer. 4, 19412,258, 36 Yonxdenne Oct. 7, 1941 2,259,372 isle;l Oct. 14, 19412,297,498 ann et al. Sept. 29. 1942 2,435,435 Fonda. Feb. 3, 19482.452.529 Laterina oct. 26. 194s 2.473.220 .x June 14, 1949 2,476,019July 19. 1949 Certificate of Correction Reissue No. 23,802 March 16,1954 Edward Emanuel Sheldon It, is hereby certified the; error appearsin the printed specification of the above numbered patent requiringeorrecion as follows:

Column 3, line 18, for electrodemagnetic read electro-magnetic oolumn 5,line 11, for 3" read i3 sind that the said Letters Patent should be readas Corrected above so that the same may conform to the record of the:use in the Patent Office.

Signed and sealed this 25th day of May, A. D. 1954.

ARTHUR Wn CROCKER,

Assistant Commissioner of Patente.

